Field of the Invention
This invention relates to hydraulic cylinders of the type in which a ram is hydraulically operated to raise and lower a particular load. In particular, the invention provides for a unique hydraulic cylinder in which the ram is enclosed within a guide cylinder which provides further support of the load and protects the ram of the hydraulic cylinder from extreme environmental conditions. The hydraulic cylinder of the present invention is provided with a unique safety locking means which can support the hydraulic cylinder and the load at any desired position and prevents the hydraulic cylinder and its load from lowering in the event of a sudden loss in hydraulic pressure.
One of the problems associated with hydraulic cylinders in which the hydraulic fluid activates a load lifting ram concerns instances in which the ram is large enough to act as a structural support column, in which instances the required safety factors against side loading necessitates the use of large diameter rams with resulting large volumes of hydraulic fluid at low operating pressures. Conventional small hydraulic cylinders exert large forces even though such cylinders are quite small. The large lifting force applied by these small hydraulic cylinders is possible simply by elevating the operating pressure. These cylinders must be externally supported against side loads and bending and are typically used on graders, dumpsters, back-hoes, etc., which operate normally in the 2,000 to 3,000 PSI range. Such relatively small hydraulic cylinders do not utilize the large volumes of hydraulic fluid required by a hydraulically operated ram also acting as a structural column to lift and support a load. The use of large volumes of hydraulic fluid to operate the load lifting ram is costly and creates unpredictable and non-uniform operation. For example, frequent applications require that the hydraulic cylinder be operated at a uniform speed during the raising and return stroke of the power lifting ram. If, for instance, it is necessary to remove the load after it has been raised, the pressure on the power lifting cylinder decreases drastically. The first requirement is to support the load during the raising operation, requiring large volumes of fluid pumped at a constant capacity and pressure. However, during the return stroke, the hydraulic pressure required is decreased causing difficulties in moving the large quantities of oil in and out of the cylinder to provide uniform movement. While pumps are available to supply fluid at the volumes and pressure required during the raising stroke, it is difficult to buy valves with the proper operating capability to permit the large quantity of fluid to flow from the cylinder at the decreased pressures when the lift is being lowered without the attached load. Consequently, the speed of the lift can be guaranteed during the working stroke, i.e., when the load is being raised, however, guaranteeing the speed on the down or return stroke without the attached load can be very difficult and subject to other problems. For instance, the viscosity of the hydraulic fluid has a tremendous bearing on the flow rate and when the hydraulic cylinder is operated under the low pressures, the viscosity of the fluid can cause serious problems in providing uniform raising and lowering of the ram.
Ordinarily, persons are not normally working under or around loads supported by small hydraulic cylinders, or at least in a position to be injured in case the loads slip and fall in the event of a sudden hydraulic pressure loss. In instances in which the ram also acts as a structural column, however, the rams are not ordinarily supported externally. Furthermore, the loads are not necessarily centered on the load platform or support means. Such a situation makes it necessary that primary consideration during the operation of the hydraulic cylinders be given to the structural strength or integrity of the ram, its guiding system and the connection between the ram and the load, and not so much as to the operating pressures. Accordingly, there is a constant fear present for any user of hydraulic equipment that there will be a failure of the valving, the cylinder itself or the hydraulic piping, to the extent that there will be an uncontrollable movement of the hydraulic cylinder. This fear is particularly felt in those areas where large and/or critical loads are being raised. For example, hydraulic elevators are susceptible to this type of problem as well as other types of equipment where people work directly under loads supported by hydraulic cylinders and/or situations where large and valuable pieces of equipment must be raised to variable heights and supported in that position for extended periods of time.
Also, there are instances in which the operating environment impairs the efficient and safe operation of the hydraulic cylinder. Extreme temperature, corrosive surroundings and atmospheres ladened with particulate matter can severely damage the power lifting ram causing work stoppages and hazardous operation.
Accordingly, there is a need for a hydraulic cylinder that will permit raising the load at a controllable pressure and will generate within the hydraulic system adequate pressure to move the hydraulic fluid through the valves at a controllable rate when the ram is being lowered. Further, a need exists for a heavy-duty hydraulic cylinder which will support the power lifting ram and attached loads at variable heights for extended periods of time, providing complete safety at all times regardless of the size of the load or its position throughout the ram stroke. A still further need involves the protection of the power lifting ram from severe conditions.